Multiple Frequency Offsets Compensation in OFDMA Femtocells

Introduction

Femtocells are small low power base stations operating in the licensed spectrum to enhance the indoor coverage and offload traffic from the macrocell networks. In order for the coexisting femtocell and macrocell networks to function, interference between these two networks should be properly controlled and considerable attention has been paid to this issue. With the properties of interference avoidance and the robustness against multipath channels, Orthogonal Frequency Division Multiple Access (OFDMA) has been considered as a promising solution to this interference control problem (Kim et al., 2009; Lόpez-Pérez et al., 2009). In OFDMA femtocells, available subcarriers are partitioned into a number of disjoint subsets and these subsets are allocated to different users to transmit data simultaneously. By properly allocating the subcarriers between macrocells and femtocells, the inter-network and intra-network interference could be perfectly mitigated.

As a multi-carrier transmission technique, OFDMA however is highly vulnerable to the Carrier Frequency Offset (CFO) caused by transceiver oscillator mismatches, and/or Doppler shifts. Even a small mismatch can destroy the orthogonality among the subcarriers and introduce Inter-Carrier Interference (ICI) and Multi-User Interference (MUI), resulting in serious performance degradation. Accordingly, frequency synchronization is essential for the OFDMA femtocells to function. In the uplink of OFDMA femtocells, the received signal is a superposition of signals transmitted from multiple users through different channels. Since the users are usually located at different places and equipped with different oscillators, each user will experience its own CFO. Due to the presence of multiple CFOs, frequency synchronization becomes challenging in the femtocell uplink. In general, frequency synchronization includes CFO estimation and compensation. Multiple CFO estimation for the OFDMA femtocell uplink has been extensively discussed (Beek et al., 1999; Cao et al., 2004; Morelli et al., 2007; Chen et al., 2008) and the focus of this chapter is on CFO compensation, which is to eliminate the ICI and MUI induced by CFOs and recover the orthogonality among subcarriers and users.

Generally, there are two kinds of methods to counteract the detrimental effect of multiple CFOs in the femtocell uplink. One is sending back the estimated CFOs to the users for them to adjust their oscillators (Beek et al., 1999). This method is simple but not applicable in the mobile environments where Doppler shifts also cause CFOs and the adjustments may be dated due to delays. In addition, feedback signaling is needed and thus reduces the system throughput. The other is compensating for the effect of the CFOs directly on the received signals at the base station. This kind of base-station-side methods is attractive for mobile environments and therefore gains a lot of attention in recent years.

In the literature, a number of base-station-side CFO compensation algorithms have been proposed (Choi et al., 2000; Huang & Letaief, 2005; Seyedi & Saulnier, 2005; Tsai et al., 2005; Marabissi et al., 2006; Sreedhar & Chockalingam, 2006; Cao et al., 2007; Lee & Kim, 2007; Monahar et al., 2007; Yucek & Arslan, 2007; Hou & Ko, 2008; Hsu & Wu, 2008). Some of them are only applicable to the systems with specific subcarrier allocations, such as interleave or block subcarrier allocations. However, in OFDMA femtocells, a Generalized Subcarrier Allocation Scheme (GCAS) is adopted, i.e., the subcarriers could be arbitrarily allocated to the users such that frequency diversity is achieved by assigning each subcarrier to the user with the best channel condition. Compensation algorithms suitable for the GCAS are thus preferable for femtocell applications.